This invention relates to hybrid vehicle powertrains having an internal combustion engine and an electric motor as sources of torque wherein engine torque is supplemented with motor torque.
The fuel economy of an automotive vehicle powertrain having an internal combustion engine can be greatly improved by supplementing engine driving torque with the torque of an electric motor. Such hybrid drivelines also reduce undesirable exhaust gas emissions from the internal combustion engine in comparison to an automotive vehicle driveline in which the engine is the sole source of driving torque.
A hybrid vehicle powertrain of this kind is described, for example, in copending U.S. patent application Ser. No. 09/567,373, filed May 9, 2000, entitled xe2x80x9cMotor/Alternator With Integral Wet Clutch For Use In Hybrid Vehiclesxe2x80x9d. Another example of a hybrid vehicle powertrain may be seen by referring to copending U.S. patent application Ser. No. 09/353,290, filed Jul. 15, 1999, entitled xe2x80x9cTransmission And Control System For Use With An Engine In A Hybrid Electric Vehiclexe2x80x9d. Both of these applications are assigned to the assignee of the present invention.
As in the case of the hybrid vehicle powertrains of the copending applications, the electric motor used with the control strategy of the powertrain of the present invention is an induction motor that develops torque to complement engine torque thereby providing added launch performance. Further, during vehicle coast-down, regenerative braking is achieved, which improves fuel economy and makes it possible for the vehicle battery power source for the motor to be charged.
In most conventional automotive vehicle powertrains, a torque converter is used between a multiple-ratio gear system and an internal combustion engine to provide added torque for acceleration and to function as a fluid coupling to attenuate engine torque fluctuations during acceleration as well as during steady-state operation. The control strategy of the present invention makes it possible to attenuate inertia forces in the powertrain during acceleration without the necessity for using a hydrokinetic torque converter. It also makes it possible to effect driving torque management by attenuating the inertia forces during transitions from an electric motor drive mode to an internal combustion engine drive mode.
The parallel torque flow path of the hybrid driveline of the invention makes it possible for the internal combustion engine to be disconnected from the driveline whenever the vehicle stops, or coasts, or operates under low power driving conditions. The electric motor thus can be used in place of the internal combustion engine to drive the vehicle. Energy lost during braking, as mentioned above, also can be partially recovered as the motor is driven by the traction wheels as it functions as a generator for charging the vehicle battery.
The electric motor is available to power the vehicle in parallel relationship with respect to the engine thereby boosting the effective power available during heavy throttle operation.
A friction clutch is located between the electric motor and the internal combustion engine. The strategy of the present invention makes it possible to achieve a smooth transition between the electric motor drive mode and the internal combustion engine drive mode, thereby minimizing disturbances due to inertia forces. The transition of power between the electric motor and the internal combustion engine can be achieved while the vehicle is in motion.
The clutch located between the engine and the electric motor may be used as well to start the internal combustion engine under cold-start operating conditions or when the vehicle battery state of charge is insufficient to provide an acceptable electric motor launch.
A powertrain that is controlled by the strategy of the present invention includes a pressure-actuated engine clutch between the engine crankshaft and the rotor of a starter motor/alternator. A spring damper is located between the engine and the clutch. During normal driving, the vehicle can be launched electrically using the motor/alternator only. At this time, the engine friction clutch is open, which disconnects the engine from the powertrain. A transition from the electric motor operating mode to the internal combustion engine operating mode is achieved by energizing the clutch. This connects the internal combustion engine to the rotor of the electric motor, which is directly coupled to the transmission input shaft. At this time, the clutch pressure is controlled in accordance with the control strategy of the present invention. Clutch pressure modulation is obtained using this strategy to provide for a smooth power transfer with minimal disturbance due to inertia forces.
During this pressure modulation, the engine clutch will continue to slip until the engine and the electric motor are at nearly the same speed. At that time the engine clutch pressure can be increased, thereby fully applying the clutch to transmit full output torque of the internal combustion engine to the transmission. At the outset, the electric motor accelerates the engine up to the engine startup speed. When the engine startup speed is achieved, the fuel is turned on; but this occurs only after the clutch has been fully applied.
The natural frequency of the spring mass system that comprises the inertia of the rotor, the inertia of the engine, and the spring damper characteristic occurs at speeds that are below the idle speed of the engine. The clutch then operates in a slip mode which prevents damper rattle and large friction clutch torque spikes as the clutch is being engaged. The clutch torque capacity, therefore, need not be designed to accommodate high peak torques.
In accordance with another feature of the invention, the internal combustion engine can be started and the vehicle can be launched from a standing start without using the electric drive system. This is useful when the battery is in a low state of charge. Under these circumstances, the engine may remain in neutral until the engine has reached its idle speed and the driver has moved the transmission selector lever to a drive position. Significant cyclical crankshaft torque reversals during the engine starting mode are avoided and the previously described damper rattle problem will not be present.
The electric motor may act as an inertial starter. In these circumstances, the electric motor freely accelerates up to a target idle speed for the engine. Then the full hydraulic pressure for the clutch is applied following a continuously slipping clutch startup mode. Torque spikes are attenuated just as they are during the previously described transition from the electric motor drive mode to the internal combustion engine drive mode. After the engine reaches its target idle speed and the engine clutch is fully applied, the fuel for the engine can be turned on. The vehicle then can operate using conventional internal combustion engine power.